1. Field of the Invention
The present invention relates to a MIS (Metal Insulator Semiconductor) FET (Field Effect Transistor), and more particularly, to an improvement of a MISFET including a GaAs substrate.
2. Description of the Background Art
Logical elements utilizing a GaAs-MES (Metal Semiconductor) FET or a GaAs-J (Junction) FET had a problem that the logic voltage swing could not be increased, because an increase in logic voltage swing will cause forward current to flow via a gate junction. If a GaAs-MISFET is used, the aforementioned problem will not occur, logically, because the gate electrode is insulated by a gate insulating film, allowing the formation of a high speed logical element that can permit to a great logic voltage swing. The usage of a GaAs-MISFET also allows the formation of a high frequency analog element that can have a signal with a large voltage swing applied to the gate electrode.
FET is known
In the conventional art, a GaAs-MISFET is known including a gate insulating film made of a material such as SiO.sub.2, SiN.sub.x, or CaF.sub.2 formed on a GaAs substrate by thermal CVD (Chemical Vapor Deposition), plasma-enhanced CVD, photo CVD, or sputtering, and a gate electrode formed thereabove. However, the use of SiO.sub.2, SiN.sub.x, or CaF.sub.2 as a gate insulating film cause an undesired increase in interface state density on account of the interface structure between the gate insulating film and the GaAs substrate being disturbed. It was therefore difficult to manufacture a practical GaAs-MISFET.
A pseudo-MISFET is known in the conventional art using an undoped A(GaAs film formed on a GaAs substrate by the MBE (Molecular Beam Epitaxy) deposition method or by the MO (Metal Organic) CVD method substituting a gate insulating film. Although this pseudo-MISFET has a good gate interface with a low interface state density, the AlGaAs film does not establish complete lattice matching with the GaAs substrate. Another disadvantage is that the band offset between the valance band of the gate insulating film and the valence band of the GaAs substrate (i.e., an energy barrier) can not be increased because of the insufficient energy band gap in the AlGaAs film. It was therefore difficult to realize a p channel MISFET.
A MISFET is known in the conventional art using a II-VI group compound semiconductor such as ZnS, ZnSe, or ZnSSe as a gate insulating film. Although ZnS and ZnSe both do not establish lattice matching with a GaAs substrate, the lattice constant of ZnSe is close to that of GaAs to allow a good gate interface by reducing significantly the thickness of the ZnSe film. Furthermore, if the ternary compound ZnSSe is used as a gate insulating film, lattice matching can be obtained between the gate insulating film and the GaAs substrate in the composition of ZnS.sub.0.06 Se.sub.0.94 to result in a good gate interface. These II-VI group compound semiconductors have a relatively great energy band gap.
However, the band offset amount at the conduction band side between the gate insulating film formed of the aforementioned II-VI group compound semiconductor and the GaAs substrate can not be increased, so that the aforementioned II-VI group compound semiconductors are not practical as a gate insulating film in a GaAs-MISFET.
The GaAs-MISFET of the conventional art had the problems of undesired high interface energy level between the interface of a gate insulating film and a GaAs substrate and insufficient band offset. A practical GaAs-MISFET was not yet obtained.